One very important industrial process is the extraction of vegetable oil from oil-bearing seeds or kernels such as soybeans, cottonseed, canola, and rapeseed. The process generally operates continuously in very large equipment, where a single unit typically extracts many tons per day of the oil. The oil is very valuable, and has many food and non-food uses. The particle mass remaining after the oil removal is also valuable, and may be used as human food or animal feed.
One type of extraction system first processes the oil-containing portion of the seeds to form a mass of flakes or particles bearing the oil (meal). Then the meal is transported to a container where a solvent such as hexane dissolves the oil in the meal. Much of the solvent-oil solution so formed is then removed from the meal by draining. The process then separates the oil and solvent removed from the meal by distillation for example, allowing the oil to be used as desired, and the solvent reused.
Hexane and other similar solvents are highly flammable, so the processes used must avoid any possibility of igniting the hexane. Hexane and other solvents also form vapors much heavier than air and water vapor, so solvent vapors tends to settle at the bottom of any vessel containing them.
The meal after the first solvent-oil removal step still has so much solvent that the meal is unfit for use as animal feed or human food. To correct this situation, a “desolventizer-toaster” (DT) unit may remove a large percentage of the remaining solvent from the meal. This leaves the remaining meal with a small amount of residual solvent. The solvent that the DT unit extracts from the meal can be reused in the process as well, making the process more cost-efficient and environmentally friendly.
A DT unit passes the meal through a number of heating stages that vaporize nearly all of the solvent remaining in the meal. Each stage comprises a floor or tray that heats the meal and/or allows steam to pass through the meal, in either case vaporizing a portion of the hexane or other solvent in the meal. A stifling element at each stage agitates the meal to assist the vaporization and to eventually shift the meal to an opening in the stage's floor through which the meal falls under the force of gravity to the next stage.
Each stage can remove only a percentage of the solvent remaining in the meal. DT units having a reasonable number of stages, say 6-10, do not remove as much of the solvent as desired to provide meal with a suitably small amount of solvent.
Certain newer DT units now have one or more solvent extraction flash stages at the bottom of the conventional heating stages that use a different process to extract a further percentage of the entrained solvent, meanwhile reusing a portion of the steam. This type of DT unit is explained in both U.S. Pat. No. 6,279,250 ('250) and in an article in Inform, June 2003, pp. 338-339 (Inform). Both '250 and Inform are incorporated by reference into this description.
It will be helpful for the reader of this description to be familiar with both of these publications. Such a solvent extraction and steam reuse stage form a Vapor Recovery (VR) enhancement of a DT unit.
This VR stage uses an ejector or other vapor transport device that collects steam and leaked solvent vapor from the rotary valve receiving meal discharged from the final conventional stage, see '250. The ejector recycles this steam and leaked solvent vapor back into an upper conventional stage of the DT unit. The stages at and below the stage receiving the recycled steam can reuse the thermal energy of the recycled steam rather than losing it. The heat in the recycled vapor will heat the meal to extract further solvent while again passing through the conventional stages, thereby reducing solvent lost to the environment and providing more solvent for reuse.
A DT unit having VR usually has only one VR stage as shown in '250. Some have however, been built with two or more VR stages in order to recover more of the solvent. Experience shows though, that in DT units with multiple VR stages, it is difficult to assure the most efficient venting of vapors from the plurality of VR stages. That is, it is difficult to find the optimum amount of steam and vapor to recycle from above the first VR stage and how much to recycle from above the second VR stage.
These conventional VR stages do not deal with the problem of pooling or gathering of the heavy solvent vapors due to inadequate agitating of the gasses in the space involved. Hexane for example, has a specific gravity that is more than five times that of water. If the steam and solvent vapors do not thoroughly mix, the heavier solvent vapors settle in the space and eventually exit the DT unit with the meal.
Experience also shows that conventional VR systems leave a small percentage of the solvent remaining in the meal. While this remaining solvent is not considered to affect the quality of the meal, it is still wasted. It would be advantageous to extract a further portion of this remaining solvent, for reuse if for no other reason.